1. Field of the Invention
The present invention relates to an image compression circuit, an image compression method, a semiconductor device, and a smartphone, particularly to an image compression circuit for compressing pieces of image data of four adjacent pixels, an image compression method, a semiconductor device, and a smartphone.
2. Description of the Related Art
Recently, in a display device, an amount of information on image data to be processed increases with increasing resolution of an image, and sometimes image processing of comparing the image data of a preceding frame and the image data of a current frame is performed in order to improve image quality of a moving image. Therefore, in order to perform the image processing of comparing the image data of the preceding frame and the image data of the current frame, it is necessary that a display driver that displays the image on the display device include a memory in which the image data of the preceding frame can be stored.
However, with increasing resolution of the image, the amount of information on the image data of one frame increases, and a capacity of the memory of the display driver is enlarged. In the case that there is a restriction to a chip size of the display driver, the capacity of the memory of the display driver cannot be ensured, and the image data of one frame cannot be stored.
Therefore, in order to store the image data of one frame in the memory, the display driver includes an image compression circuit that compresses the image data. In the image compression circuit, the image data is compressed such that the image data of each pixel expressed by an RGB color space is converted into the image data of each pixel expressed by a YCbCr color space. The image compression circuit can also be used as an image processing device for a game machine in addition to the display driver. For example, in an image processing device disclosed in Japanese Patent Laying-Open No. 2011-15139, an image compression circuit compresses the image data stored in a sprite pattern memory in the case of overlay display in which a sprite image, such as a game character, is superimposed on a background image.
However, unfortunately the image quality of the compressed image data is degraded when the image compression circuit simply compresses the image data by converting the image data of each pixel expressed by the RGB color space into the image data of each pixel expressed by the YCbCr color space.
In the image compression circuit, there is also a problem in that a compression rate of the image data is lowered to increase the information amount of the compressed image data when the degradation of the image quality of the compressed image data is suppressed.